This invention relates to a control device for a direct-coupling hydraulic clutch provided in a hydraulic torque converter of an automative vehicle, and more particularly to a control device of this kind which is adapted to control the operation of the direct-coupling hydraulic clutch at changeover of the transmission gear ratio or gear shifting of a transmission coupled to the hydraulic torque converter.
A direct-coupling hydraulic clutch in a hydraulic torque converter of an automotive vehicle is already known, which is adapted to mechanically lock up the torque converter to reduce the slip loss to a minimum required value when the amplification of torque performed by the torque converter is almost not available, to thereby curtail the fuel consumption during driving of the vehicle. However, at gear-shifting operation of a transmission which is coupled to the torque converter in line therewith, the locking-up of the torque converter should be temporarily interrupted so that shocks caused upon gear-shifting desirably are absorbed by the torque converter. To this end, a control device for the direct-coupling hydraulic clutch has been proposed by the assignee of the present application, which is equipped with a selector valve operable in response to the pressure of operating oil outputted from an oil hydraulic servo mechanism which establishes a certain speed reduction ratio of the transmission, to assume one of first and second operating positions to be assumed, respectively, before and after establishment of the speed reduction ratio, or a third or transitional position intervening between the first and second operating positions. In the first or second position, the selector valve communicates the direct-coupling clutch with an operating oil source which supplies the operating oil, while in the third position, it communicates the direct-coupling clutch with a tank for returning the operating oil.
However, it is sometimes required to arrange, together with the above proposed selector valve, various other control valves in a fluid line extending between the selector valve and the direct-coupling clutch for the convenience of designing the hydraulic circuit, such control valves including a modulator valve which controls the engaging force of the direct-coupling clutch in response to the vehicle speed and/or the throttle valve opening, so as to release the torque converter from the completely locked-up state in a particular engine operating region to allow slippage within the torque converter for absorption of engine vibrations occurring in such operating region, and an idle relief valve which interrupts the locking-up of the torque converter upon detection of the full closing of the throttle valve, to thereby ensure smooth driving of the vehicle while the engine is at idle. To arrange the various control valves in the fluid line between the selector valve and the clutch necessitates prolonging the fluid line, resulting in increased flow resistance in the fluid line. Due to this increased flow resistance as well as to flow resistance of the above control valves per se, there takes place pressure loss in the operating oil supplied to the direct-coupling clutch, causing a response lag in the operation of the direct-coupling clutch, which makes it difficult to interrupt the locking-up of the torque converter in immediate response to gear-shifting which usually is effected within a very short period of time. This difficulty becomes serious particularly when the viscosity of operating oil is high, such as at the start of the engine in cold weather. Therefore, it is desirable to interrupt engagement of the direct-coupling clutch as immediately as possible after the gear-shifting operation has been started.
On the other hand, after the gear-shifting operation has been completed, reengagement of the direct-coupling clutch should be effected with a proper time delay. An electronic control device which electronically controls such a direct-coupling clutch would be capable of easily setting the timing of reengagement of the direct-coupling clutch after the gear-shifting operation so as to provide a suitable time delay between the completion of the gear-shifting operation and the reengagement of the clutch. However, such electronic control device in general is expensive.